I make dump filters for all of my big tanks (and small ones too). Basically a DIY sump (rubbermaid) on top of the tank...fed by a pump in the tank...and water dumps back into the tank via gravity (holes in the bottom of the rubbermaid).
Because you're not losing much head (from pumping water up several feet as with a traditional, under-the-tank sump) and because the dropping of the water via gravity moves a lot of water, you can get away with using a smaller pump. You also don't need a drilled tank.
You can also feed the dump filter with your canister. Basically set the return into the dump filter...and voila!
I'm filtering a heavily stocked 180 with an FX5 that feeds an 18g dump filter (filled with bio balls). That's it (other than a couple of small air-fed filters).
I'm filtering a 150g with a pond sponge filter...and a 350gph pump feeding a dump filter.
It's energy efficient, works great and easy to clean. The only drawback is that it's not exactly something that I'd want in my living room (unless I used I nice rubbermaid or concealed it under a canopy)...or my bedroom (sound of falling water)...
Matt
